| ReentrantLock.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util.concurrent.locks;
9 import java.util.*;
10 import java.util.concurrent.*;
11 import java.util.concurrent.atomic.*;
12
13 /**
14 * A reentrant mutual exclusion {@link Lock} with the same basic
15 * behavior and semantics as the implicit monitor lock accessed using
16 * {@code synchronized} methods and statements, but with extended
17 * capabilities.
18 *
19 * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last
20 * successfully locking, but not yet unlocking it. A thread invoking
21 * {@code lock} will return, successfully acquiring the lock, when
22 * the lock is not owned by another thread. The method will return
23 * immediately if the current thread already owns the lock. This can
24 * be checked using methods {@link #isHeldByCurrentThread}, and {@link
25 * #getHoldCount}.
26 *
27 * <p>The constructor for this class accepts an optional
28 * <em>fairness</em> parameter. When set {@code true}, under
29 * contention, locks favor granting access to the longest-waiting
30 * thread. Otherwise this lock does not guarantee any particular
31 * access order. Programs using fair locks accessed by many threads
32 * may display lower overall throughput (i.e., are slower; often much
33 * slower) than those using the default setting, but have smaller
34 * variances in times to obtain locks and guarantee lack of
35 * starvation. Note however, that fairness of locks does not guarantee
36 * fairness of thread scheduling. Thus, one of many threads using a
37 * fair lock may obtain it multiple times in succession while other
38 * active threads are not progressing and not currently holding the
39 * lock.
40 * Also note that the untimed {@link #tryLock() tryLock} method does not
41 * honor the fairness setting. It will succeed if the lock
42 * is available even if other threads are waiting.
43 *
44 * <p>It is recommended practice to <em>always</em> immediately
45 * follow a call to {@code lock} with a {@code try} block, most
46 * typically in a before/after construction such as:
47 *
48 * <pre>
49 * class X {
50 * private final ReentrantLock lock = new ReentrantLock();
51 * // ...
52 *
53 * public void m() {
54 * lock.lock(); // block until condition holds
55 * try {
56 * // ... method body
57 * } finally {
58 * lock.unlock()
59 * }
60 * }
61 * }
62 * </pre>
63 *
64 * <p>In addition to implementing the {@link Lock} interface, this
65 * class defines methods {@code isLocked} and
66 * {@code getLockQueueLength}, as well as some associated
67 * {@code protected} access methods that may be useful for
68 * instrumentation and monitoring.
69 *
70 * <p>Serialization of this class behaves in the same way as built-in
71 * locks: a deserialized lock is in the unlocked state, regardless of
72 * its state when serialized.
73 *
74 * <p>This lock supports a maximum of 2147483647 recursive locks by
75 * the same thread. Attempts to exceed this limit result in
76 * {@link Error} throws from locking methods.
77 *
78 * @since 1.5
79 * @author Doug Lea
80 */
81 public class ReentrantLock implements Lock, java.io.Serializable {
82 private static final long serialVersionUID = 7373984872572414699L;
83 /** Synchronizer providing all implementation mechanics */
84 private final Sync sync;
85
86 /**
87 * Base of synchronization control for this lock. Subclassed
88 * into fair and nonfair versions below. Uses AQS state to
89 * represent the number of holds on the lock.
90 */
91 static abstract class Sync extends AbstractQueuedSynchronizer {
92 private static final long serialVersionUID = -5179523762034025860L;
93
94 /**
95 * Performs {@link Lock#lock}. The main reason for subclassing
96 * is to allow fast path for nonfair version.
97 */
98 abstract void lock();
99
100 /**
101 * Performs non-fair tryLock. tryAcquire is
102 * implemented in subclasses, but both need nonfair
103 * try for trylock method.
104 */
105 final boolean nonfairTryAcquire(int acquires) {
106 final Thread current = Thread.currentThread();
107 int c = getState();
108 if (c == 0) {
109 if (compareAndSetState(0, acquires)) {
110 setExclusiveOwnerThread(current);
111 return true;
112 }
113 }
114 else if (current == getExclusiveOwnerThread()) {
115 int nextc = c + acquires;
116 if (nextc < 0) // overflow
117 throw new Error("Maximum lock count exceeded");
118 setState(nextc);
119 return true;
120 }
121 return false;
122 }
123
124 protected final boolean tryRelease(int releases) {
125 int c = getState() - releases;
126 if (Thread.currentThread() != getExclusiveOwnerThread())
127 throw new IllegalMonitorStateException();
128 boolean free = false;
129 if (c == 0) {
130 free = true;
131 setExclusiveOwnerThread(null);
132 }
133 setState(c);
134 return free;
135 }
136
137 protected final boolean isHeldExclusively() {
138 // While we must in general read state before owner,
139 // we don't need to do so to check if current thread is owner
140 return getExclusiveOwnerThread() == Thread.currentThread();
141 }
142
143 final ConditionObject newCondition() {
144 return new ConditionObject();
145 }
146
147 // Methods relayed from outer class
148
149 final Thread getOwner() {
150 return getState() == 0 ? null : getExclusiveOwnerThread();
151 }
152
153 final int getHoldCount() {
154 return isHeldExclusively() ? getState() : 0;
155 }
156
157 final boolean isLocked() {
158 return getState() != 0;
159 }
160
161 /**
162 * Reconstitutes this lock instance from a stream.
163 * @param s the stream
164 */
165 private void readObject(java.io.ObjectInputStream s)
166 throws java.io.IOException, ClassNotFoundException {
167 s.defaultReadObject();
168 setState(0); // reset to unlocked state
169 }
170 }
171
172 /**
173 * Sync object for non-fair locks
174 */
175 final static class NonfairSync extends Sync {
176 private static final long serialVersionUID = 7316153563782823691L;
177
178 /**
179 * Performs lock. Try immediate barge, backing up to normal
180 * acquire on failure.
181 */
182 final void lock() {
183 if (compareAndSetState(0, 1))
184 setExclusiveOwnerThread(Thread.currentThread());
185 else
186 acquire(1);
187 }
188
189 protected final boolean tryAcquire(int acquires) {
190 return nonfairTryAcquire(acquires);
191 }
192 }
193
194 /**
195 * Sync object for fair locks
196 */
197 final static class FairSync extends Sync {
198 private static final long serialVersionUID = -3000897897090466540L;
199
200 final void lock() {
201 acquire(1);
202 }
203
204 /**
205 * Fair version of tryAcquire. Don't grant access unless
206 * recursive call or no waiters or is first.
207 */
208 protected final boolean tryAcquire(int acquires) {
209 final Thread current = Thread.currentThread();
210 int c = getState();
211 if (c == 0) {
212 if (isFirst(current) &&
213 compareAndSetState(0, acquires)) {
214 setExclusiveOwnerThread(current);
215 return true;
216 }
217 }
218 else if (current == getExclusiveOwnerThread()) {
219 int nextc = c + acquires;
220 if (nextc < 0)
221 throw new Error("Maximum lock count exceeded");
222 setState(nextc);
223 return true;
224 }
225 return false;
226 }
227 }
228
229 /**
230 * Creates an instance of {@code ReentrantLock}.
231 * This is equivalent to using {@code ReentrantLock(false)}.
232 */
233 public ReentrantLock() {
234 sync = new NonfairSync();
235 }
236
237 /**
238 * Creates an instance of {@code ReentrantLock} with the
239 * given fairness policy.
240 *
241 * @param fair {@code true} if this lock should use a fair ordering policy
242 */
243 public ReentrantLock(boolean fair) {
244 sync = (fair)? new FairSync() : new NonfairSync();
245 }
246
247 /**
248 * Acquires the lock.
249 *
250 * <p>Acquires the lock if it is not held by another thread and returns
251 * immediately, setting the lock hold count to one.
252 *
253 * <p>If the current thread already holds the lock then the hold
254 * count is incremented by one and the method returns immediately.
255 *
256 * <p>If the lock is held by another thread then the
257 * current thread becomes disabled for thread scheduling
258 * purposes and lies dormant until the lock has been acquired,
259 * at which time the lock hold count is set to one.
260 */
261 public void lock() {
262 sync.lock();
263 }
264
265 /**
266 * Acquires the lock unless the current thread is
267 * {@linkplain Thread#interrupt interrupted}.
268 *
269 * <p>Acquires the lock if it is not held by another thread and returns
270 * immediately, setting the lock hold count to one.
271 *
272 * <p>If the current thread already holds this lock then the hold count
273 * is incremented by one and the method returns immediately.
274 *
275 * <p>If the lock is held by another thread then the
276 * current thread becomes disabled for thread scheduling
277 * purposes and lies dormant until one of two things happens:
278 *
279 * <ul>
280 *
281 * <li>The lock is acquired by the current thread; or
282 *
283 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
284 * current thread.
285 *
286 * </ul>
287 *
288 * <p>If the lock is acquired by the current thread then the lock hold
289 * count is set to one.
290 *
291 * <p>If the current thread:
292 *
293 * <ul>
294 *
295 * <li>has its interrupted status set on entry to this method; or
296 *
297 * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
298 * the lock,
299 *
300 * </ul>
301 *
302 * then {@link InterruptedException} is thrown and the current thread's
303 * interrupted status is cleared.
304 *
305 * <p>In this implementation, as this method is an explicit
306 * interruption point, preference is given to responding to the
307 * interrupt over normal or reentrant acquisition of the lock.
308 *
309 * @throws InterruptedException if the current thread is interrupted
310 */
311 public void lockInterruptibly() throws InterruptedException {
312 sync.acquireInterruptibly(1);
313 }
314
315 /**
316 * Acquires the lock only if it is not held by another thread at the time
317 * of invocation.
318 *
319 * <p>Acquires the lock if it is not held by another thread and
320 * returns immediately with the value {@code true}, setting the
321 * lock hold count to one. Even when this lock has been set to use a
322 * fair ordering policy, a call to {@code tryLock()} <em>will</em>
323 * immediately acquire the lock if it is available, whether or not
324 * other threads are currently waiting for the lock.
325 * This "barging" behavior can be useful in certain
326 * circumstances, even though it breaks fairness. If you want to honor
327 * the fairness setting for this lock, then use
328 * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
329 * which is almost equivalent (it also detects interruption).
330 *
331 * <p> If the current thread already holds this lock then the hold
332 * count is incremented by one and the method returns {@code true}.
333 *
334 * <p>If the lock is held by another thread then this method will return
335 * immediately with the value {@code false}.
336 *
337 * @return {@code true} if the lock was free and was acquired by the
338 * current thread, or the lock was already held by the current
339 * thread; and {@code false} otherwise
340 */
341 public boolean tryLock() {
342 return sync.nonfairTryAcquire(1);
343 }
344
345 /**
346 * Acquires the lock if it is not held by another thread within the given
347 * waiting time and the current thread has not been
348 * {@linkplain Thread#interrupt interrupted}.
349 *
350 * <p>Acquires the lock if it is not held by another thread and returns
351 * immediately with the value {@code true}, setting the lock hold count
352 * to one. If this lock has been set to use a fair ordering policy then
353 * an available lock <em>will not</em> be acquired if any other threads
354 * are waiting for the lock. This is in contrast to the {@link #tryLock()}
355 * method. If you want a timed {@code tryLock} that does permit barging on
356 * a fair lock then combine the timed and un-timed forms together:
357 *
358 * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
359 * </pre>
360 *
361 * <p>If the current thread
362 * already holds this lock then the hold count is incremented by one and
363 * the method returns {@code true}.
364 *
365 * <p>If the lock is held by another thread then the
366 * current thread becomes disabled for thread scheduling
367 * purposes and lies dormant until one of three things happens:
368 *
369 * <ul>
370 *
371 * <li>The lock is acquired by the current thread; or
372 *
373 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
374 * the current thread; or
375 *
376 * <li>The specified waiting time elapses
377 *
378 * </ul>
379 *
380 * <p>If the lock is acquired then the value {@code true} is returned and
381 * the lock hold count is set to one.
382 *
383 * <p>If the current thread:
384 *
385 * <ul>
386 *
387 * <li>has its interrupted status set on entry to this method; or
388 *
389 * <li>is {@linkplain Thread#interrupt interrupted} while
390 * acquiring the lock,
391 *
392 * </ul>
393 * then {@link InterruptedException} is thrown and the current thread's
394 * interrupted status is cleared.
395 *
396 * <p>If the specified waiting time elapses then the value {@code false}
397 * is returned. If the time is less than or equal to zero, the method
398 * will not wait at all.
399 *
400 * <p>In this implementation, as this method is an explicit
401 * interruption point, preference is given to responding to the
402 * interrupt over normal or reentrant acquisition of the lock, and
403 * over reporting the elapse of the waiting time.
404 *
405 * @param timeout the time to wait for the lock
406 * @param unit the time unit of the timeout argument
407 * @return {@code true} if the lock was free and was acquired by the
408 * current thread, or the lock was already held by the current
409 * thread; and {@code false} if the waiting time elapsed before
410 * the lock could be acquired
411 * @throws InterruptedException if the current thread is interrupted
412 * @throws NullPointerException if the time unit is null
413 *
414 */
415 public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
416 return sync.tryAcquireNanos(1, unit.toNanos(timeout));
417 }
418
419 /**
420 * Attempts to release this lock.
421 *
422 * <p>If the current thread is the holder of this lock then the hold
423 * count is decremented. If the hold count is now zero then the lock
424 * is released. If the current thread is not the holder of this
425 * lock then {@link IllegalMonitorStateException} is thrown.
426 *
427 * @throws IllegalMonitorStateException if the current thread does not
428 * hold this lock
429 */
430 public void unlock() {
431 sync.release(1);
432 }
433
434 /**
435 * Returns a {@link Condition} instance for use with this
436 * {@link Lock} instance.
437 *
438 * <p>The returned {@link Condition} instance supports the same
439 * usages as do the {@link Object} monitor methods ({@link
440 * Object#wait() wait}, {@link Object#notify notify}, and {@link
441 * Object#notifyAll notifyAll}) when used with the built-in
442 * monitor lock.
443 *
444 * <ul>
445 *
446 * <li>If this lock is not held when any of the {@link Condition}
447 * {@linkplain Condition#await() waiting} or {@linkplain
448 * Condition#signal signalling} methods are called, then an {@link
449 * IllegalMonitorStateException} is thrown.
450 *
451 * <li>When the condition {@linkplain Condition#await() waiting}
452 * methods are called the lock is released and, before they
453 * return, the lock is reacquired and the lock hold count restored
454 * to what it was when the method was called.
455 *
456 * <li>If a thread is {@linkplain Thread#interrupt interrupted}
457 * while waiting then the wait will terminate, an {@link
458 * InterruptedException} will be thrown, and the thread's
459 * interrupted status will be cleared.
460 *
461 * <li> Waiting threads are signalled in FIFO order.
462 *
463 * <li>The ordering of lock reacquisition for threads returning
464 * from waiting methods is the same as for threads initially
465 * acquiring the lock, which is in the default case not specified,
466 * but for <em>fair</em> locks favors those threads that have been
467 * waiting the longest.
468 *
469 * </ul>
470 *
471 * @return the Condition object
472 */
473 public Condition newCondition() {
474 return sync.newCondition();
475 }
476
477 /**
478 * Queries the number of holds on this lock by the current thread.
479 *
480 * <p>A thread has a hold on a lock for each lock action that is not
481 * matched by an unlock action.
482 *
483 * <p>The hold count information is typically only used for testing and
484 * debugging purposes. For example, if a certain section of code should
485 * not be entered with the lock already held then we can assert that
486 * fact:
487 *
488 * <pre>
489 * class X {
490 * ReentrantLock lock = new ReentrantLock();
491 * // ...
492 * public void m() {
493 * assert lock.getHoldCount() == 0;
494 * lock.lock();
495 * try {
496 * // ... method body
497 * } finally {
498 * lock.unlock();
499 * }
500 * }
501 * }
502 * </pre>
503 *
504 * @return the number of holds on this lock by the current thread,
505 * or zero if this lock is not held by the current thread
506 */
507 public int getHoldCount() {
508 return sync.getHoldCount();
509 }
510
511 /**
512 * Queries if this lock is held by the current thread.
513 *
514 * <p>Analogous to the {@link Thread#holdsLock} method for built-in
515 * monitor locks, this method is typically used for debugging and
516 * testing. For example, a method that should only be called while
517 * a lock is held can assert that this is the case:
518 *
519 * <pre>
520 * class X {
521 * ReentrantLock lock = new ReentrantLock();
522 * // ...
523 *
524 * public void m() {
525 * assert lock.isHeldByCurrentThread();
526 * // ... method body
527 * }
528 * }
529 * </pre>
530 *
531 * <p>It can also be used to ensure that a reentrant lock is used
532 * in a non-reentrant manner, for example:
533 *
534 * <pre>
535 * class X {
536 * ReentrantLock lock = new ReentrantLock();
537 * // ...
538 *
539 * public void m() {
540 * assert !lock.isHeldByCurrentThread();
541 * lock.lock();
542 * try {
543 * // ... method body
544 * } finally {
545 * lock.unlock();
546 * }
547 * }
548 * }
549 * </pre>
550 *
551 * @return {@code true} if current thread holds this lock and
552 * {@code false} otherwise
553 */
554 public boolean isHeldByCurrentThread() {
555 return sync.isHeldExclusively();
556 }
557
558 /**
559 * Queries if this lock is held by any thread. This method is
560 * designed for use in monitoring of the system state,
561 * not for synchronization control.
562 *
563 * @return {@code true} if any thread holds this lock and
564 * {@code false} otherwise
565 */
566 public boolean isLocked() {
567 return sync.isLocked();
568 }
569
570 /**
571 * Returns {@code true} if this lock has fairness set true.
572 *
573 * @return {@code true} if this lock has fairness set true
574 */
575 public final boolean isFair() {
576 return sync instanceof FairSync;
577 }
578
579 /**
580 * Returns the thread that currently owns this lock, or
581 * {@code null} if not owned. When this method is called by a
582 * thread that is not the owner, the return value reflects a
583 * best-effort approximation of current lock status. For example,
584 * the owner may be momentarily {@code null} even if there are
585 * threads trying to acquire the lock but have not yet done so.
586 * This method is designed to facilitate construction of
587 * subclasses that provide more extensive lock monitoring
588 * facilities.
589 *
590 * @return the owner, or {@code null} if not owned
591 */
592 protected Thread getOwner() {
593 return sync.getOwner();
594 }
595
596 /**
597 * Queries whether any threads are waiting to acquire this lock. Note that
598 * because cancellations may occur at any time, a {@code true}
599 * return does not guarantee that any other thread will ever
600 * acquire this lock. This method is designed primarily for use in
601 * monitoring of the system state.
602 *
603 * @return {@code true} if there may be other threads waiting to
604 * acquire the lock
605 */
606 public final boolean hasQueuedThreads() {
607 return sync.hasQueuedThreads();
608 }
609
610
611 /**
612 * Queries whether the given thread is waiting to acquire this
613 * lock. Note that because cancellations may occur at any time, a
614 * {@code true} return does not guarantee that this thread
615 * will ever acquire this lock. This method is designed primarily for use
616 * in monitoring of the system state.
617 *
618 * @param thread the thread
619 * @return {@code true} if the given thread is queued waiting for this lock
620 * @throws NullPointerException if the thread is null
621 */
622 public final boolean hasQueuedThread(Thread thread) {
623 return sync.isQueued(thread);
624 }
625
626
627 /**
628 * Returns an estimate of the number of threads waiting to
629 * acquire this lock. The value is only an estimate because the number of
630 * threads may change dynamically while this method traverses
631 * internal data structures. This method is designed for use in
632 * monitoring of the system state, not for synchronization
633 * control.
634 *
635 * @return the estimated number of threads waiting for this lock
636 */
637 public final int getQueueLength() {
638 return sync.getQueueLength();
639 }
640
641 /**
642 * Returns a collection containing threads that may be waiting to
643 * acquire this lock. Because the actual set of threads may change
644 * dynamically while constructing this result, the returned
645 * collection is only a best-effort estimate. The elements of the
646 * returned collection are in no particular order. This method is
647 * designed to facilitate construction of subclasses that provide
648 * more extensive monitoring facilities.
649 *
650 * @return the collection of threads
651 */
652 protected Collection<Thread> getQueuedThreads() {
653 return sync.getQueuedThreads();
654 }
655
656 /**
657 * Queries whether any threads are waiting on the given condition
658 * associated with this lock. Note that because timeouts and
659 * interrupts may occur at any time, a {@code true} return does
660 * not guarantee that a future {@code signal} will awaken any
661 * threads. This method is designed primarily for use in
662 * monitoring of the system state.
663 *
664 * @param condition the condition
665 * @return {@code true} if there are any waiting threads
666 * @throws IllegalMonitorStateException if this lock is not held
667 * @throws IllegalArgumentException if the given condition is
668 * not associated with this lock
669 * @throws NullPointerException if the condition is null
670 */
671 public boolean hasWaiters(Condition condition) {
672 if (condition == null)
673 throw new NullPointerException();
674 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
675 throw new IllegalArgumentException("not owner");
676 return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
677 }
678
679 /**
680 * Returns an estimate of the number of threads waiting on the
681 * given condition associated with this lock. Note that because
682 * timeouts and interrupts may occur at any time, the estimate
683 * serves only as an upper bound on the actual number of waiters.
684 * This method is designed for use in monitoring of the system
685 * state, not for synchronization control.
686 *
687 * @param condition the condition
688 * @return the estimated number of waiting threads
689 * @throws IllegalMonitorStateException if this lock is not held
690 * @throws IllegalArgumentException if the given condition is
691 * not associated with this lock
692 * @throws NullPointerException if the condition is null
693 */
694 public int getWaitQueueLength(Condition condition) {
695 if (condition == null)
696 throw new NullPointerException();
697 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
698 throw new IllegalArgumentException("not owner");
699 return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
700 }
701
702 /**
703 * Returns a collection containing those threads that may be
704 * waiting on the given condition associated with this lock.
705 * Because the actual set of threads may change dynamically while
706 * constructing this result, the returned collection is only a
707 * best-effort estimate. The elements of the returned collection
708 * are in no particular order. This method is designed to
709 * facilitate construction of subclasses that provide more
710 * extensive condition monitoring facilities.
711 *
712 * @param condition the condition
713 * @return the collection of threads
714 * @throws IllegalMonitorStateException if this lock is not held
715 * @throws IllegalArgumentException if the given condition is
716 * not associated with this lock
717 * @throws NullPointerException if the condition is null
718 */
719 protected Collection<Thread> getWaitingThreads(Condition condition) {
720 if (condition == null)
721 throw new NullPointerException();
722 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
723 throw new IllegalArgumentException("not owner");
724 return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
725 }
726
727 /**
728 * Returns a string identifying this lock, as well as its lock state.
729 * The state, in brackets, includes either the String {@code "Unlocked"}
730 * or the String {@code "Locked by"} followed by the
731 * {@linkplain Thread#getName name} of the owning thread.
732 *
733 * @return a string identifying this lock, as well as its lock state
734 */
735 public String toString() {
736 Thread o = sync.getOwner();
737 return super.toString() + ((o == null) ?
738 "[Unlocked]" :
739 "[Locked by thread " + o.getName() + "]");
740 }
741 }
742